Traditional telephone networks comprise switched networks, e.g., the public switching telephone network (PSTN) and ISDN lines, in which a physical link path is established between the end users of a call. The voice signals of the call are passed, generally in a time domain multiplexed (TDM) manner, on the physical link path. On the other hand, data networks, such as the Internet, are generally packet based networks.
Due to the wide availability of switched network lines, these lines are commonly used to access packet based networks. Data connections passed on switched network lines are referred to as voice band modem (VBM) connections. At opposite ends of a VBM connection, modems translate data signals into voice signals and translate voice signals from the telephone network into data signals. The tasks of the modems are generally divided into a plurality of layers. From a top down view, a first layer performs data compression (DC) tasks. In transmission, the DC layer receives data bits and provides compressed data bits. In reception, the DC layer receives compressed data bits and provides uncompressed data bits. The compression is used to conserve bandwidth and is optional. The data compression is performed, for example, in accordance with the V.44 or V.42bis ITU recommendation or the MNP5 standard.
A second layer, referred to as an error correction (EC) layer, performs tasks which are generally divided into two modules, a link access procedure for modem (LAPM) module, and a high level data link control (HDLC) module. In transmission, the tasks of the LAPM module receive a stream of compressed bits (if compression is used), break the stream into frames and add to each frame a type, sequence number and an acknowledgment field. In reception, the LAPM tasks acknowledge the receipt of the frames and send the transmitting modem indications on the amount of unused space in the buffer of the receiving modem.
The tasks of the HDLC module, in transmission, add an error correction field (e.g., CRC) to the transmitted frames and pad the frame with flags (e.g., 0x7E bytes), according to the transmission rate of the modem, such that the number of bits transmitted in each time interval is constant. In reception, the HDLC module removes padding flags and the error correction field and discards frames with an erroneous CRC.
A third layer, referred to as a data pump (DP) layer, performs modulation and demodulation, i.e., converts data bits into voice symbols and vice versa.
Data transmitted on a VBM connection is generally handled in accordance with the DC layer, EC layer and DP layer by the transmitting modem and by the DP layer, EC layer and DC layer by the receiving modem. Thus, the same layers are applied at both end modems of the VBM connection, although the receiving modem substantially reverses the operations of the transmitting modem.
Although most modems perform the tasks of all three layers described above, the term modem covers apparatus which performs DP tasks even if the other tasks are not performed by the apparatus.
VBM connections may be established between client modems, which are connected to a PSTN through twisted pairs, between server modems which are connected directly to the infrastructure of the PSTN, or between a server modem and a client modem. The various connections may be established in accordance with various standards, such as the V.22, V.32, V.34, V.90, V.91 and V.92 ITU recommendations.
In establishing a VBM connection, a pair of modems, on opposite ends of the line, perform a negotiation procedure in which the modems test the line and determine operation parameters, e.g., a data rate and/or protocol according to which data will be transferred over the connection. Generally the negotiation procedure includes a plurality of stages, such as one or more DP, EC and/or DC stages. Generally, the DP negotiation stages include four negotiation stages, referred to also as phases 1-4, which include a protocol selection stage (e.g., according to the V.8 protocol), and negotiation stages for training the DP, selection of symbol and/or bit rates and determination of a round trip delay. The EC and DC negotiation stages include, for example, an EC protocol negotiation (V.42 or MNP5) stage and an ECDC parameter negotiation stage (e.g., selection of a compression format).
The negotiation stages are performed in accordance with protocols which state which signals are to be transmitted at what times. Some of the signals must be transmitted within a short interval from the reception of a control signal from the other party to the negotiation. Therefore, negotiating modems are generally connected over channels which have a round trip delay beneath a required threshold.
After the negotiation stages, a data transfer stage is performed at which the modems modulate and demodulate the data packets they are provided and pass the modulated signals over the connection. During both the negotiation stage and the data transfer stage, signals are transmitted on the connection in both directions at the same time, in an operation mode referred to as full duplex. Generally, the pair of modems includes a call modem which initiates the formation of the connection and an answer modem which responds to the initiative of the call modem.
During the data transfer stage, the modems employ provisions to cancel far-echo effects. In addition, the EC layer in each of the modems verifies, as described above, that all the data transmitted was properly received and if necessary requires re-transmissions. For these tasks, the modems generally measure the round trip delay (RTD) of signals transmitted between them during the negotiation stage of VBM connections, for example during the second phase of the negotiation stage of the V.34 protocol. Similar methods to those described in the V.34 for determining the RTD are described in other protocols, such as the V.90.
According to the V.34 protocol, at a specific time, the answer modem initiates a phase reversal of a signal A it transmits. The call modem responds with a phase reversal in a B signal it transmits, 40 msec after detecting the phase reversal in the A signal. Thereafter, the answer modem responds with a second phase reversal, 40 msec after detection of the phase reversal in the B signal. Accordingly, the call and answer modems independently calculate the RTD of the connection.
The telephone network is also used for fax connections. In fax connections, image data is modulated onto voice signals, which are transmitted over switched networks together with control information. Before transmission of signals, fax connections include a test stage in which the cap abilities of the connection are determined. The signals on the fax connection, both during the test stage and during the data transmission, are transmitted in only one direction in an operation mode referred to as half duplex. That is, at any time only one end unit fax machine transmits signals on the connection.
Existing PSTN hardware cannot support the increasing demand for all types of communication services, and therefore additional hardware is added to the PSTN instead of, or in addition to, the current hardware of the PSTN. In many cases, packet based networks are cheaper to install and maintain than switched networks. Therefore, telephony providers are adding many packet based lines and/or networks to their infrastructure, especially to their long distance infrastructure.
An exemplary hybrid telephone connection includes two switched segments which connect end units, e.g., telephone sets, to respective gateways. The gateways, on the other hand, are connected through a packet based network. The gateways receive streams of signals from the respective end units, pack the streams, for example in a pulse code modulation (PCM) format into packets and pass the packets over the packet based network to the other gateway. The other gateway unpacks the packets into a stream of signals which is forwarded to the other end unit.
The travel time of packets through the packet based network may vary for different packets of a single connection. Therefore, the gateways maintain jitter buffers which delay the packet they receive over the packet based network for a short period allowing the gateway to organize the packets in their original order. When packets are not received within the short delay, the gateways generate filler signals to replace the data in the delayed packet. If only a small percentage of packets are delayed beyond the allowed period (or are otherwise lost), telephone conversations are passed with sufficient quality such that the loss of packets is substantially unnoticed.
In some cases, fax signals are handled by gateways in the same way voice signals are handled. However, the loss of a relatively small percent of the modulated signals is sufficient to prevent demodulation of the signals. Therefore, in some gateways, when the gateway receives fax signals, the gateway demodulates the signals and passes them to the other gateway in a predetermined packet format, for example as described in the T.38 ITU-T recommendation, “Procedures for real-time Group 3 facsimile communication over IP networks”, the disclosure of which is incorporated herein by reference. Gateways generally identify the fax signals using methods such as described, for example, in U.S. Pat. No. RE35,740, the disclosure of which is incorporated herein by reference. The other gateway extracts the Fax data from the packets and re-modulates the Fax data for transmission to its respective end-unit.
It is noted that most VBM connections are local connections to Internet service providers (ISP), which provide gateway services to packet-based networks. Nonetheless, there are cases when it is desired to create long distance modem connections, for example, for remote access (RAS) applications.